One of the most important minerals in biological tissue is magnesium. Many diseases can exist where magnesium-ion concentration is abnormally high or low. For example, decreased serum-magnesium ion levels may be encountered in various types of malnutrition, acute pancreatitis, alcoholism with delirium tremens, chronic glomerulonephritis, hyperthyroidism, hypoparathyroidism, hyperaldosteronism, and severe diarrhea. Serum-magnesium levels above the normal range may be found in uremia, vitamin D intoxication, and in pituitary dwarfs treated with human-growth hormone.
Of significant importance is magnesium-deficiency tetany, which has especially been found in alcoholic patients. It is characterized by low serum-magnesiumion and normal serum-calcium levels. Probably the most important application for magnesium determinations in serum is in the treatment of this disease.
The most useful methods available for determining the concentration of magnesium ions in bodily fluids include atomic absorption, colorimetric methods, fluorometric methods, and by the precipitation of magnesium ammonium phosphate with the subsequent determination of phosphate by any of a variety of methods. Of these, atomic absorption spectrophotometry is the most accurate method, although it may be necessary to add lanthanum or strontium to overcome interferences. It requires, however, expensive instrumentation and relatively large samples.
Direct determination of magnesium-ion concentration may be made colorimetrically with the use of magnesiumindicator dyes, such as methylthymol blue, Titan Yellow (methylbenzothiazide-1,3-4,4'-diazo aminobenzol-2,2'-disulfonic acid), Calmagite [1-(1-hydroxy-4-methyl-2-phenalazo)-2-naphthol-4-sulfonic acid] and Magon [1-azo-2-hydroxy-3-(2,4-dimethylcarboxanilido)naphthalene-1(2-hydroxybenze ne)]. For example, magnesium ions form a red lake with the dye Titan Yellow in an alkali solution. The absorbence of the solution against a standard is taken at the appropriate wavelength and the concentration of magnesium calculated according to Beer's Law. The method is simple and fast, but has accuracy only to within about 10 percent. Furthermore, the method follows Beer's Law for only a short range, thereby making it necessary to run multiple standards with each determination. In addition, there may be interferences in the serum sample, for example, from patients receiving calcium glucamate or mercurial diuretics, that lessen the accuracy of the method.
U.S. Pat. No. 3,754,864 describes a method using Calmagite which is simple and accurate but requires a long incubation time (20 minutes), and the working reagent is stable for only 24 hours.
Magnesium concentrations may also be determined by fluorometric methods. Magnesium will form a fluorescing complex with certain chelating agents such as 8-hydroxy5-quinoline sulfonic acid and o,o'-dihydroxyazobenzene. The complexes fluoresce when excited at an appropriate wavelength and the fluorescence can be measured. The method is simple and very sensitive. However, the method is not widely used, primarily because of the enhancing or quenching effect of other compounds that may be present.
There is a need for a simple, fast and inexpensive method for the determination of magnesium ions in serum and other biological fluids.